1. Field of the Invention
The present invention relates to a fixation control device for controlling the operation of a fixing unit in an image forming apparatus such as an electrostatic copying machine.
2. Description of Related Arts
Recently, electrostatic copying machines capable of making a copy of a large-size document original such as of JIS A0 size have become available. "JIS A0 size", which is one of the sheet sizes (finished dimensions) specified by Japanese Industrial standards, is 841 mm.times.1189 mm.
Since it is difficult to handle large-size copy sheet such as of A0 size one by one, a roll sheet is generally used which is formed of an elongated continuous sheet wound around a roll core and installed in a copying machine.
In general, an electrostatic copying machine forms a copy image of a document original on a copy sheet in the following manner. The document original is scanned under light irradiation, and a photoreceptor is exposed to light reflected on the document original, whereby an electrostatic latent image is formed on the photoreceptor. The electrostatic latent image is developed into a toner image, which is then thermally fixed on a copy sheet in a fixing unit. Thus, a copy image of the document original is formed on the copy sheet.
The fixing unit has a rubber press roller and a heat roller heated by a heater and is adapted to thermally fix the toner image on the copy sheet passing between these rollers. The fixing unit should be heated up to a predetermined temperature for thermal fixation. In accordance with a conventional fixation temperature control method, the temperature of the heat roller is controlled to approach a predetermined temperature by detecting the temperature of the heat roller and switching on and off the heater based on the detected temperature.
Where copies are successively made on a plurality of narrow-size sheets such as of A2 size in a copying machine capable of copying a large-size document original such as of A0 size, the sheets passing between a heat roller and a press roller remove heat only from the longitudinally central portion of the heat roller and press roller and, thereafter, the temperature of the heat roller is maintained at a predetermined control temperature. However, the temperatures of the end portions of the heat roller and press roller become much higher than the control temperature because the sheets pass between the heat roller and the press roller without contacting the end portions thereof.
As a result, the diameters of the end portions of the rubber press roller are increased and thereby the contact pressure applied to the longitudinally central portion of the heat roller by the press roller is reduced. This reduces the sheet transportation ability of the heat roller and press roller as a whole.
Accordingly, a sheet is transported by the heat roller and press roller at a speed different from a speed at which the sheet is transported by transportation rollers provided on the upstream side thereof along a transportation direction, and slacks before the heat roller and press roller. This causes the sheet to be offset from an image transfer position of a photoreceptor, resulting in a transfer offset.
This problem is not limited to the aforesaid copying machine for copying a large-size document original, but may occur in a copying machine adapted to copy document originals of various sizes.